Piezoelectric motor

ABSTRACT

A piezoelectric motor achieves maximum mechanical performance, high safety of operation and low costs with a negligible exciting voltage. The piezoelectric engine has a stator whose oscillator designed as a closed waveguide is provided with at least one generator of an elastic travelling wave for the waveguide. A rotor is frictionally engaged by a functional surface of the oscillator. The elastic travelling wave generator is as long as the generated wavelength and consists of a base generator and two supplementary generators that generate elastic longitudinal stationary waves having the same amplitude and wavelength. The stationary waves are offset with respect to the wave generated by the base generator by ±1/3 of the wavelength along the waveguide. Exciting sources are associated with the base generator and the supplementary generators. The supplementary generators and their exciting sources are designed in such a way that the stationary waves they generate are phase shifted by ±2/3 π with respect to the stationary wave generated by the base generator. The length of the waveguide is the same as the wavelength of the stationary wave or is an integral multiple thereof.

BACKGROUND OF THE INVENTION

The invention relates to a piezoelectric motor, particular for use asminiature electrical motors adapted for continuous or stepped rotationalmovements. The motor according to the invention can be used inautomation systems, in robot techniques, in machine tools to preciselyposition cutters, in vehicles as screen wiper motors and pane liftermotors and as a drive means for seat adjustments. It can also be used ininertialess drive means for TV aerials and other devices which require agreat moment of rotation at a comparatively low speed of rotation.

Piezoelectric motors are known in which the electrical energy isconverted into a rotational motion of a rotor by means of piezoelectricoscillators which comprise two resonators of two different standingwaves as disclosed in the U.S. Pat. Specification No. 4,019,073, forexample. With this kind of motor it is difficult to match the twodifferent types of standing waves over a wide range of temperature aswell as under mechanical stress. Said disadvantage is not inherent inpiezoelectric motors which are based on the principle of generatingacoustic travelling waves by means of uniform standing waves (refer toAxel Froschler, Analyse eines Piezo-Wanderwellenmotors, Dissertation,Stuttgart, 1992, pg. 48 ff.). These motors are disadvantageous due tothe use of acoustic waves which require the piezoelement which generatesthe travelling wave being embodied as a thin ring cemented by an elasticorganic adhesive to a metallic waveguide.

Due to the use of elastic waves and due to the small volume of thepiezoelement, compared to the entire volume of the waveguide, theefficiency factor of the electromechanic energy conversion is low whichrequires a considerable increase of the excitation voltage for themotor. The connection between the piezoelement and the metallicwaveguide by means of an organic adhesive restricts the maximummechanical motor output, reduces its efficiency factor and thereliability. The piezoelement of a motor employing such waves exhibitsdifferently oriented polarizations which renders the manufacturedifficult and the motor expensive so that piezoelectric travelliing wavemotors cannot compete with the cheap electromagnetic motors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a piezoelectricmotor including a highly efficient oscillator which ensures, at a lowexcitation voltage, a maximum mechanical output, a high reliability, anda low price.

It is a further object to provide a piezoelectric motor in which atravelling wave is generated in a monolithic piezoelectric oscillator bygenerating acoustic longitudinal waves of equal amplitude, saidtravelling wave circularly moves a the friction face of the oscillatorunder continuous elastic contact to a rotor and applies a torque to therotor via a points moving along a closed path on the oscillator surface.

The objects are realized because the maximum feasible bulk of the motoraccording to the invention is selected from a piezoelectric material,preferably from ceramics, and is used for the generation of acousticwaves the motor exhibits an effective electromechanical coefficient,which involves a considerable reduction of the excitation voltage.Additionally, the solution suggested by the invention omits cementingthe piezoelement to the waveguide by adhesives which predominantlyconsist of organic components. This permits a maximum mechanical stresson the oscillator of the motor, only limited by the stability of theceramics, and, hence, a maximum mechanic output. The piezoelement of themotor is unidirectionally polarized which simplifies its manufacture incontrast to comparable electromagnetic motors and renders itinexpensive.

The present invention can be realized by different modifications. In,for example, a simple embodiment of the invention it is feasible tocoordinate two independent generators for standing waves to a thirdgenerator depending on the two first generators, the third generatorbeing controlled by the two first generators in such a manner that itcooperates with them as a voltage divider with respect to the voltagesupplied by a voltage generating unit and as an adding operator withrespect to the voltage divided by the first two generators. The threegenerators are mounted on a cylindrical, conical or disk-shapedpiezoelectric bulk. This embodiment permits a simplification of theconstruction since it does without an excitation source for the standingwave generators. A further advantageous embodiment of the invention isobtained when the closed waveguide is embodied as a cylindrical bulkmade of piezoceramics in which the standing wave generators areconstituted as sections of parallel electrodes. With this modificationthe entire oscillator bulk operates as the piezoelectric active medium,and such a motor exhibits a feasible maximum electromechanicalcoefficient. In this case, the electric excitation voltage for the motoris a minimum. Since the oscillator design omits any cemented jointsusing organic adhesives, a maximum permissible mechanical stress can beproduced in the oscillator, only limited by the material strength of thewaveguide. This yields a maximum of oscillating motions of the points onthe operation face of the oscillator resulting, in turn, in a maximummechanical output at the motor axle. The absence of any cemented jointusing organic adhesives reduces the mechanical losses within theoscillator and enhances the efficiency. Additionally, the reliability ofthe motor is increased, since a decomposition of cemented joints at ahigh excitation level is eliminated. With a further advantageousembodiment the closed waveguide is in shape of a passive cylindricalbulk made of metal or ceramics, the standing wave generators beingembodied in such a manner that packets of piezoelectric transducers arearranged upon them. In this case the oscillator of the motor can haveany desired dimensions. The latter are not limited by manufacturingtechnology for the piezoelectric cylinders. Thus it feasible to producepiezoelectric power motors of some kilowatt output.

According to the invention the operation face of the oscillator ispreferably provided with a thin wear-resistant friction layer being inoperational contact with the rotor. This embodiment permits the frictionparameters of the operation face of the oscillator to be set whichenhances the rotation moment of the motor.

In one embodiment of the invention the friction layer can be entirelymade of a material forming a chemical compound with the piezoelectricceramics such as glass, metal, or any other suitable material. A motorin which the friction layer forms a chemical compound with theoscillator nondestructively operates up to the maximum permissiblemechanical stress of the oscillator. With a further advantageoussolution the friction layer is constituted of a base layer and anintermediate layer. The base determines the friction properties and theintermediate layer forms a chemical compound with the piezo material.Such an embodiment considerably multiplies the number of suitablefriction materials. In the subsequent modification the frictional layerconsists of a mixture of material based upon a material which forms achemical compound with the piezoceramics and, as a filler, uses amaterial which raises the friction coefficient of the friction layer.With this embodiment the desired wave resistance of the friction layercan be selected by varying the ratio between the materials used. Instill another embodiment the friction layer can be made of a porousmaterial exhibiting a high friction coefficient and a high mechanicalstrength, the pores of said material being filled with a materialforming a chemical compound with the piezoceramics. In this manner it isfeasible to manufacture considerably solid friction layers which ensurea long run of the motor in step operation.

Advantageously, power amplifiers are employed for the electricalcircuitry of the piezoelectric motor, said amplifiers being connectedvia a phase shifting chain to the excitation source of the basicgenerator and being the excitation sources for boosters. Thisembodiments permits a sharp tuning of the generators over a wide rangeas concerns temperature as well as mechanical load which improves thereliability of the inventional motor.

Furthermore, in a preferable embodiment, it is feasible to provide theexcitation sources of the boosters with means for reversal of the phaseangle of the signal. This permits a reversal of the rotation directionof the rotor. According to the invention it is also feasible toconstitute the basic generator by a frequency controlled voltagegenerator which has one frequency input. Such a modification permits asteady control of the excitation source frequency.

In a further modification the piezoelectric motor is embodied in such amanner that the basic generator has a positive feedback of the standingwaves and to its excitation source and, in combination, constitute anelectromechanical autogenerator, the frequency of the standing wavegenerators always being in the range of the resonance frequency of theoscillator, thus enhancing the rotation frequency stability of therotor.

In an embodiment of the motor it is feasible to connect the positivefeedback branch to an impedance element which, in turn, isseries-connected to the standing wave basic generator. It is alsofeasible to connect the positive feedback branch to a currenttransformer series-connected to the standing wave basic generator.

Finally, the positive feedback branch can be connected to the feedbackelectrode which, due to the standing waves produced by the generator, isarranged at the locus where the maximum mechanical stress occurs.

The last three embodiments mentioned ensure a safe excitation of theelectromechanical generator at the oscillator resonance frequency. Afurther advantageous embodiment of the motor is provided with anelectronic circuit for disconnecting the positive feedback branch. Inthis case a step control can easily be carried out which permitsapplication for step-motors.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention become obvious from the followingdescriptions of the Figures. By virtue of the schematical drawings theinvention will be explained in more detail by thirty-four embodiments.

The Figures show in:

FIG. 1 a perspective view of the substantial components of a motor ofthe present invention.

FIG. 2 a schematic view of an oscillator,

FIG. 3 an excitation voltage diagram,

FIG. 4 a standing wave deformation of an oscillator with an excitationvoltage applied,

FIG. 5 a diagram of an excitation voltage,

FIG. 6 an instantaneous state of an operational face of the oscillatorwhen generating a standing wave,

FIG. 7 diagrams for representing superimposition process when deformingthe operation face,

FIG. 8 a complete representation of the path of travel of points on theoscillator operational face when generating a standing wave,

FIG. 9 another representation of the path of travel of points on theoscillator operational face when generating a standing wave,

FIG. 10 a representation of the development of path of travel of a pointwith the oscillator operation face,

FIG. 11 a representation of the phase movements of points of theoscillator operation face,

FIGS. 12a-12d feasible lines of movement of points on the operationface,

FIGS. 13a-13c an elementary volume of an oscillator and its equivalentcircuit diagram,

FIG. 14 a complete equivalent circuit diagram of the oscillator,

FIG. 15 a simplified equivalent circuit diagram of said oscillator,

FIGS. 16a-16f the electromechanical parameters of the motor,

FIGS. 17a-17h oscillator modifications,

FIGS. 18a-18e modifications of wear-resistant layers,

FIGS. 19a-19e are different electrode modifications,

FIG. 20 a special electrode arrangement,

FIG. 21 a nonsymmetrical travelling wave,

FIG. 22 block diagrams to explain the basic principle underlying theoscillator control,

FIG. 23 an oscillator comprising two independent generators and onegenerator depending on the latter,

FIG. 24 an equivalent electric circuit of the generators of FIG. 23,

FIG. 25 a block diagram for oscillator self-excitation,

FIG. 26 a block diagram including a frequency controlled autogenerator,

FIG. 27 a block diagram of an electromechanical autogeneratorarrangement,

FIG. 28 a block diagram of a current transformer,

FIG. 29 a feedback electrode and its operating mode,

FIG. 30 a block diagram including a feedback electrode,

FIG. 31 a block diagram of a step motor,

FIG. 32 a longitudinal section of a motor design including an oblongoscillator,

FIG. 33 a top view of the oscillator of FIG. 32,

FIG. 34 a motor design including an annular oscillator,

FIG. 35 a sectional view of a motor design in a clamp housing,

FIG. 36 a partially plan view, partially sectional view of the motordesign of FIG. 35,

FIG. 37 a motor design including packet transducers in an axial sectionof FIG. 38 along the axis XXXVII--XXXVII, FIG. 38 a plan view of FIG.37,

FIG. 39 an axial section of a motor design comprising an annular magnet,

FIG. 40 a basic circuit diagram including an frequency-controlledautogenerator,

FIG. 41 a first motor circuitry employed as electromechanicalautogenerator,

FIG. 42 a second motor circuitry employed as electromechanicalautogenerator,

FIG. 43 a basic circuit diagram of the motor of the present inventionemployed as a step-motor, and

FIG. 44 a basic circuit diagram of a motor including two independentgenerators and one generator depending on the latter.

According to FIG. 1a piezoelectric motor comprises a stator 1 includinga cylindrical oscillator 2 in the shape of a closed waveguide 3 made ofpiezoelectric ceramics. The waveguide 3 is provided with at least onegenerator 4 for producing an elastic travelling wave in the bulk of thewaveguide 3. In the embodiment shown, a rotor 6 is forced by its ownweight against a front face 5 (operation face) of the oscillator 2. Asecond non-operational face 7 of the oscillator 2 rests upon an elasticsound-isolating base 8 arranged in a not shown housing of the device. Atravelling wave generator 4 comprises a basic generator 9 and twoadditional generators (boosters) 10, 11 for producing standing waves.Each of the generators 9, 10, 11 occupies equally large sections 12, 13,14 of the cylindric wave guide 3 and is constituted of two parallelelectrodes 15, 16 with intermediate piezoelectric ceramics. Each of theelectrodes 15, 16 represents current carrying metallic layers attachedin longitudinal direction to the cylindrical wave-guide 3. To activatethe generators 9, 10, 11 the piezoelectric ceramics provided between theelectrodes 15, 16 is polarized normally relative to said electrodes15,16. In FIG. 1 and in the subsequent FIGS. the polarization isindicated by the arrows 150.

Each of the standing wave generators has an excitation source (voltagesource) 17, 18, 19. The contacts 20, 21 of each excitation source 17,18, 19 are connected to electrodes 15 and 16 of respective generators 9,10, 11. The respective voltages U₁ =U·sinωt, U₂ =U·(sin ωt±120°), U₃=U·sin(ωt±120°) are applied across the contacts 20, 21 and, thus, acrossthe electrodes 15, 16 of the standing wave generators 9, 10, 11.

In FIG. 2 a schematic of the cylindrical wave guide 3 is shown,indicating a height h and a width b. The length of the wave guide 3considered along the central line 22, has to be an integer multiple of awavelength λ being produced by the standing wave generators 9, 10, 11 inthe waveguide. The length of each standing wave generator 9, 10, 11,considered along the central line 22, is λ/3. The boosters 10, 11 areshifted by a respective amount of ±λ/3 relative to the basic generator9, considered along the central line 22. Each group of standing wavegenerators 9, 10, 11 constitutes a travelling wave generator 4. Thespace occupied by a travelling wave generator 4 along the central lineof the cylindrical waveguide 3 equals the wavelength λ. The number ofgenerators 4 can be selected as desired and depends on the length of thecentral line 22 of the waveguide 3. The waveguide 3, shown in FIG. 2,has five travelling wave generators. The five generators 4 areelectrically connected in parallel. Such a circuit means that thecontacts 20 of each excitation source 17, 18, 19 are connected to therespective electrode 16 of the generators 9, 10, 11. In FIG. 2 theconnections 21 of the excitation sources 17, 18, 19 are connected withone another and connected to the electrode 15 which is the commonelectrode of the generators 9, 10, 11. The excitation sources 17, 18, 19supply sine-shaped voltages of equal amplitude and equal frequency,phase-shifted to one another. The phase-shift of the electrical voltagesof the excitation sources 18, 19 relative to the voltage of theexcitation source 17 is ±2/3 π (±120°). FIG. 3 shows diagrams of thevoltage curves. Tile voltage U₁ is supplied from the excitation source17, the voltage U₂ from the excitation source 18, and the voltage U₃from the excitation source 19.

In FIG. 4 a longitudinal standing wave is shown in the waveguide 3 and,hence, in its operational face 5 which is produced by one of thegenerators 9, 10, 11, the respective generator being excited via asine-shaped voltage U_(i) =U·sin ωt of the respective excitation source17, 18, 19. The waveguide 3, shown in FIG. 4 has eight parallelconnected equal standing wave generators 9, 10 or 11. Said generatorsproduce a standing wave having sixteen maxima and minima, respectively.The number of equal generators 9, 10, 11 is determined by die number oftravelling wave generators 4. During operation of the motor the standingwave generators 9, 10, 11 independently produce three standing waves ofequal amplitude and frequency which are spatially shifted to one anotherby λ/3 along the central line 22 (FIG. 2) of the waveguide 3. The phasesof said waves differ from one another by the amount of ±2/3 π (±120°).

Hereinafter the basic operations of the inventional piezoelectric motorare described in more detail in connection with the FIGS. 1 to 16.Excitation voltages U₁, U₂, U₃, (FIG. 3) supplied from the respectiveexcitation sources 17, 18, 19 are applied across each standing wavegenerator 9, 10, 11 (FIGS. 1 and 2). Each voltage controls therespective generator 9, 10, 11. Said generators produce in the waveguide 3 of the oscillator 2 three identical longitudinal standing wavesof equal amplitude which are, related to the central line 22 of the waveguide 3, spatially shifted to one another by an amount of ±λ/3 and,time-dependent (phase) shifted by an angle ±2/3 π (±120°). In FIG. 4 oneof said waves is shown. Since the amplitude of oscillation of theoscillator 2 (2-10 μm) is small in relation to its dimensions (20 to 100mm) each wave is produced independently of one another. They existindependently of one another and deform the operation face 5 of theoscillator 2 independently of one another. The displacement of thepoints of the oscillator 2 substantially at right angles to theoperational face 5 varies in accordance with the sine law due to theaction of each individual wave so that the displacement of the points onthe operational face 5 follows this law and in accordance with thevariation of the sine-shaped excitation voltage. FIG. 5 shows a diagramof the voltage phase of one of the standing wave generators 9, 10, 11for the given period of time t₀ -t₈ of one oscillating period.

FIG. 6 shows the phases of motion of the operational face 5 of theoscillator 2 for the period of time t₀ -t₈. In the diagram the points a₀of the operational face 5 are indicated which are in the extreme valuesof the transversal speed of the standing waves. Said points oscillate onstraight lines (indicated by dash-points) which are at right angles tothe operational face 5. When three standing waves are simultaneouslyproduced in the oscillator 2 then they superimpose. The deformations ofthe oscillator bulk 2 caused by the standing waves sum up, so do thedeformations of the operational face 5.

In FIG. 7 the superposition operation for deforming the operational face5 is shown. The indication of the positions 23, 24, 25 corresponds tothe points of time t₁, t₂, t₃ of the instantaneous values of theexcitation voltages U₁, U₂, U₃ across the generators 9, 10, 11 (FIG. 3).In FIG. 7 the dashed lines 26, 27, 28 indicate the deformation state ofthe operational face 5 caused by each standing wave. The solid line 29characterizes the superimposed deformation of the operational face 5.From the diagrams 23, 24, 25 (FIG. 7) it is visible that thesuperimposed deformation of the operational face 5 of the oscillator 2is a travelling wave 29 of the oscillator 2. Said wave travels a pathlength in one oscillation period which corresponds to the wavelength λof the standing wave. The amplitude of the travelling wave equals the1.5-fold value of amplitude of one of the standing wives.

According to FIG. 8 two basicly different schemes of motion are feasiblefor the points on the operational face 5 of the oscillator 2. In thefirst case the points a₀ to ±a₅ move apart in the peak range when thestanding wave bulges, with the operational face 5 expanding, as it were.In the second case the points a₀ to ±a₅ as approach one another. Theoperational face is compressed. The first case may occur when theoscillator 2 is of electromechanical coupling coefficient. The electricfield forces between the oscillator electrodes 15, 16 predominantlyaffect the points of the oscillator 2. In dependence on the size andsign of the excitation field all points of the oscillator 2 depart fromor approach one another, the paths of travel of the points beinginclined curved lines or closed ellipses.

FIG. 8 shows the complete motion diagram of the points a₁ -a₅ of theoperational face 5 within the wavelength range of the waveguide 3(oscillator 2) which is λ/2 when a standing wave is generated in theoscillator 2. The motion lines of the points a₁ -a₅ (FIG. 8) areindicated by the curved dashed lines. The wave guide 3 (oscillator 2)alternatingly bulges (points ±a₁ ' to ±a₅ ') and contracts in theoscillation bulge (points ±a₁ " to ±a₅ 41 ). It is obvious from thecourse of motion of the points a₀ to a₅ shown in FIG. 8 that the pointsa₀ in the maxima of the perpendicular oscillation components move onstraight lines at right angles to the surface 5. The cylindrical waveguide 3 of the oscillator 2 represents an elastic bulk of finitedimensions, the elementary particles of which are held by one another byforce of elasticity. In consequence thereof, when a longitudinalstanding wave is excited, the points of the wave guide 3 (oscillator 2)move along inclined paths, as shown in FIG. 8, in the course of whichthey alternatingly depart from and approach one another. Only thosepoints in the extreme values move (oscillate) on straight paths at rightangles to the surface of the cylindrical wave guide 3 (refer to FIGS. 6and 8). Said points have a maximum transverse amplitude Δy_(max). Theremaining points travel along paths inclined relative to the surface ofthe wave guide as shown in FIG. 8. With an increasing distance to thecentral point a₀ their transverse amplitude (along y-direction) isreduced. This explains why each point a_(n) of the operational face 5remote from point a₀ (line) of the maximum by a certain distance has atransverse and a longitudinal component of speed. With an increasingdistance (points +a_(n) and -a_(n)) on both sides of the central pointa₀ the longitudinal component (x-direction) of speed increases and, at adistance of ±λ/4 the transverse amplitude reaches a maximum value ofΔx_(max). The transverse componente Δy is minimal in this point. Thenon-linear path of travel (curve) depends on inhomogeneities of thewaveguide material at great oscillation amplitudes, what cannot beeliminated in practice.

FIG. 9 explains the second case of motion of the points a₀ to ±a₃ on theoperational face of the oscillator 2. This case may occur at a highmechanical quality of the oscillator 2 and a low electromechanicalcoupling coefficient of the piezoceramic material of the oscillator 2.In this case the mechanical forces which compress and expand theoscillator bulk due to the motion of the acoustic wave are the mainreason for the motion of the points. In practice, the path of travel ofthe points form similitudes of ellipsoidal figures which are inclinedunder an angle towards the oscillator surface. Each of the two evaluatedcases concerning the motion of the points on the operational face of theoscillator 5 does not vary the operational principle underlying themotor; however, there is a substantial difference as concerns therotation direction of the rotor 6.

According to FIG. 10, when a travelling wave is generated in thewaveguide 3 of the oscillator 2, each point on the operational face 5 ofthe oscillator 2 subsequently passes the positions of the entire pointsfrom +a_(n) ' to -a_(n) ' and from -a_(n) " to +a_(n) ", whiletravelling along a closed path which generally is an ellipse and, in aspecial case, a circle. The direction of travel of the wave is indicatedby an arrow 151. The maximum height of the ellipse equals double themaximum transverse amplitude Δy_(max). The maximum width of the ellipseis double the maximum longitudinal amplitude Δx_(max). The phase angleincluded by the longitudinal and transverse component always is 90°.Provided that the transverse component Δy equals zero the maximumlongitudinal component Δx obtains a maximum, and when the longitudinalcomponent Δx is a maximum the transverse component Δy substantially iszero. Each point a_(n) has a course of motion on its elliptical pathdiffering from that of point a_(n) ±1. FIG. 11 represents the course ofmotion of points a₀ to a₇ on the operational face 5. It can be seen thatthe points a₀ which are, at the point of time given, on a wave peakcontact the rotor 6. Due to the friction force said points of theoperational face 5 convey a torsion moment to the rotor 6. The rotormaterial exhibits a certain constant plasticity so that said contact, inpractice, is a surface contact, the contacting face depending on theelastic properties of the material of the rotor and on the resilientforce which presses the rotor 6 to the operational face 5 of theoscillator 2. As it were, a certain number of points on the wave peak ofthe travelling wave are in contact with the rotor to transmit a torsionmoment to the latter, the points in the wave troughs moving in a reversedirection. During one oscillation period each point of the operationalface 5 entirely passes its path of travel. The number of surfaceportions of the operational face 5 contacting the rotor 6 equals thenumber of wavelengths λ corresponding to the length of the central line22 of the oscillator 2 (FIG. 13). The rotation direction of the ellipsesor the direction of travel of the points in contact to the rotor 6depends on whether or not the points in the peaks of the standing waveat the point of time of expansion of the oscillator material (FIGS. 8and 9) depart from or approach one another. When the points depart fromone another the ellipses and the rotor 6 rotate in reverse directionwith respect to the propagation direction of the travelling wave. Whenthe points approach one another the rotation direction of the ellipsesand the rotor 6 coincides with the propagation direction of tiletravelling wave. Said case is not considered here. The relation Δx/Δybetween the longitudinal and transverse component, i.e. the course ofmotion of the motor is substantially defined by the ratio of thewavelength λ to the height h of the waveguide. The greater said ratio isthe more the ellipses are expanded along the operational face 5. Thesmaller said ratio is the more the path of travel of points approaches acircular path. The width of the oscillator (wave guide) and theelasticity properties of the oscillator material are of low influence onthe ratio of the components Δx/Δy. With motors in which the path oftravel of the points of the operational face 5 approximates a circularshape the ratio of wavelength λ to the height of oscillator h is about2. FIGS. 12a-12d show feasible paths of travel of points on theoperational face 5, namely, FIG. 12a shows an oblong ellipse at rightangles to the operation face 5, FIG. 12b shows a circular path oftravel, FIG. 12c shows an oblong ellipse of an axial ratio of 1.5extending along the operational face 5, and FIG. 12d shows an ellipsehaving an axial ratio of 4. A path of travel in shape of an oblongellipse extending along the operational face 5 and having a ratio ofcomponents within the range of 2 to 5, FIGS. 12c and 12d, is optimal asconcerns operation of the motor. In this case the energy flow directedtangentially to the operational face 5 is four to twenty-five timesgreater than that directed at right angles relative to said face; thatis, the greater portion of stator energy potentially is converted intorotor energy. The circular path of travel in FIG. 12b is a criticalcase. As concerns the oblong ellipse extending at right angles to theoperational face 5, FIG. 12a, the main portion of energy of the stator 2enters into the rotor 6 of the motor, excites the latter and isconverted there into heat. That is the reason why such a path of travelis undesired. In the case of an extremely oblong ellipse (not shown)extending along the operational face 5 the energy directed at rightangles to the rotor face would not suffice to establish a frictioncontact between oscillator 2 and rotor 6. With such motors a largeportion of energy would be released, inter alia, from the oscillator 2as heat. Accordingly, the oscillator 2 of the motor has an optimumgeometry which is determined by the path of travel of the points. Saidgeometry is determined by the ratio from the length of the standing waveλ generated to the height h of the oscillator 2 (waveguide 3). The exactvalue for the optimum ratio depends on the elasticity properties of theoscillator 2 material and from its width b and lies within a range offrom λ to 0.25 λ. Independently from one another the standing wavegenerators 9, 10, 11 in the oscillator 3 generate acoustic standingwaves so that said generators can be considered as an independentelectromechanical oscillation system. Therefore it is feasible to selectan elementary volume of a length λ for each generator 9, 10, 11 on theoscillator 2 including a standing wave generator as shown in FIG. 13a.Such a volume can be considered as an elementary independent oscillationsystem of the length λ with electrodes of the length λ/2, i.e., as atwo-mode piezoelectric resonator as shown in FIG. 13b. The equivalentelectric circuit of such a resonator is shown in FIG. 13c. The circuitcomprises the following components:

a static capacity

C_(o) =s/b·.di-elect cons.₃₃ ^(T) (1-K₃₁ ²), wherein s is the area ofthe electrodes.

an ideal electromechanical transducer having a transducer coefficient of

N_(p) =b/2·d₃₁ /S₁₁ ^(E)

a mechanical capacity

C_(M) =λ/π² ·S₁₁ ^(E) /b·h

a mechanical inductivity

L_(M) =m, wherein m is the mass of the elementary volume,

a resistor for the mechanical oscillator losses

R_(M) =ωL_(M) /θ_(e).G., wherein θ_(e).G is the effective quality of themotor.

The complete equivalent circuit of a transducer generator 4 can beconsidered as a unit of three identical equivalent circuits 37, 38, 39(FIG. 14) of the standing wave generators 9, 10, 11. In the completecircuit each of the equivalent circuits 37, 38, 39 is connected to therespective excitation source 17, 18, 19. The output of each equivalentcircuits 37, 38, 39 is connected to the friction transducer 40. Thefriction transducer 40 given in the complete equivalent circuit reflectsthe friction contact of the piezoelectric motor. It comprises aresistance for the friction losses R_(F) and an ideal transducer fortransducing the oscillations of the oscillator 2 into a rotationalmotion of the rotor 6, wherein

N_(F) =M/F.sub.˜, M=constant torsion moment,

F.sub.˜ =a variable force,

is valid for the transducing coefficient.

The output of the friction transducer 40 is connected to the resistanceof the mechanical load R_(L).

The equivalent circuit shown in FIG. 14 can be reduced into a simplerone as shown in FIG. 15, provided that the piezoelectric motor operatesat the mechanical resonant frequency of the oscillator 2. In saidequivalent circuit the parameters U₁ ', U₂ ', U₃ ', R_(M) ', R_(F) ',are transferred to the mechanical part of the circuit according to FIG.14. The motor exhibits the conventional electromechanic properties ofpiezoelectric motors. In practice the resistor R_(M) for the mechanicallosses in the oscillator generally is considerably smaller than theimpedance ωL_(M) of the oscillator 5. Accordingly, the frequencydependence of the motor exhibits characteristics shown in FIG. 16a-16c.FIG. 16a shows the rotation frequency n as a function of the excitationfrequency f. FIG. 16b shows the phase current I_(F) (current of one ofthe standing wave generators 9, 10, 11) as a function of the excitationfrequency f, and FIG. 16c shows the phase shift φ as a function of theexcitation voltage U₁ (U₂, U₃) and of the phase current I_(F). Allcharacteristic curves of FIGS. 16a-16c are interconnected via typicalpoints. Hence, the maximum of the rotation frequency n_(max) correspondsto the mechanical resonant frequency of the oscillator f₀ (ω₀). Saidfrequency is indicative of a zero-phase shift between the input voltageU₁ (U₂, U₃) and the input current I_(F) of each of the phases as shownin FIG. 16c. The maximum current I_(Fmax) lies at the frequency f_(Imax)left from the mechanical resonant frequency f₀. The current function 42has a second zero phase at the anti-resonant frequency f_(a) of theoscillator 6. The phase shift between the input voltages U₁ (U₂, U₃) andthe phase current I_(F) (43) is positive in the range of from f₀ tof_(a). It is negative in the remaining ranges of the excitationfrequency. All three functions are continuous ones, that is, there areno ranges of non-defined states. FIG. 16d, plots a function of thecontrol voltage, i.e. the dependence of the rotation frequency n on thelevel of the excitation voltage U₁ (U₂, U₃) when the motor operates atthe mechanic resonant frequency f₀. This function is substantiallylinear with a small discontinuity at the voltage U₀ when the transverseamplitude of the oscillations of the oscillator 2 is smaller than theroughness of the operational face 5. FIG. 16e mechanical characteristicvalues of the motor are plotted, the dependence of the rotationfrequency n on the load moment M when the motor operates at themechanical resonant frequency is shown. The function is linear up to theload moment M_(e). At said moment the friction contact breaks off. Inthe load range of from M_(e) to M_(max) the motor operatesdiscontinuously. The dashed line indicates the possible maximum value ofthe load moment M'_(max). This value is attainable at an ideal selectionof material for the friction coupling of stator l and rotor. FIG. 16fshows the quality diagram of the motor, that is, the dependence of theefficiency factor η on the load moment M when the motor operates at themechanical resonant frequency f₀. The function follows a parabolic curveup to the load value M_(e). The maximum M.sub.ηmax lies in the vicinityof half the possible maximum load moment 1/2M'_(max). The size of themaximum efficiency coefficient η_(max) depends on the relation betweenthe loss resistance R'_(M) of the oscillator and the loss resistance ofthe friction contact R'_(F). In a physical realization the lossresistance of the friction contact R'_(F) is always greater than theloss resistance of the oscillator R'_(M). Hence, the motor has asufficiently great efficiency which lies in a range of from 30% to 70%.

FIG. 17a-17h shows modifications of monolithic oscillators 2 of themotor entirely manufactured of piezoceramics. Said oscillators are thewaveguides for the travelling waves produced in the generator 4. Each ofthe shown modifications can be employed in dependence on the physicalconstructions and the required motor parameters. Accordingly, theoscillator 2, in FIG. 17a for example, embodied as an oblong cylinderthe height of which is greater than or equal to its diameter, permitsthe construction of piezoelectric motors of minimum dimensions includinga travelling wave generator 4 (FIG. 1). The oscillator 2 embodied as ashort broad cylinder, in FIG. 17b, is employed when a maximum mechanicaloutput is required. In this case the diameter of the oscillator 2 can beextended at will, the number of travelling wave generators 4 and,accordingly, the number of waves contacting the rotor 6 is increased. Adisk-shaped oscillator in FIG. 17c, which also can be considered as acylinder, can be employed as a flat motor of minimum height. When apiezoelectric motor of minimum cross-section and a central opening isrequired, an annular, shown in FIG. 17d oscillator is employed The useof an oscillator of conical cross-section, shown in FIGS. 17e and 17f,permits a multiplication of the oscillation speed. Such oscillators workin analogy to concentrators for mechanical tensions which multiply theoscillation speed in a narrow range of their cross-section. The use ofmultiplying concentrators permits a 1.5-fold to a twofold increase ofthe oscillation speed and, hence, of the rotation frequency of therotor. FIGS. 17g and 17h shows oscillators having conical operationalfaces 5. Such oscillators permit the construction of piezoelectricmotors without centering ball-bearings which considerably reduces thecosts of the motors. Since the conical operational face 5 of theoscillator 2 is a large one the mechanical output of the motor isenhanced in relation thereto.

The piezoelectric motor is based upon a friction contact of theoscillator 2 to the rotor 6, i.e. the faces acting one upon the otherare subject to a friction wear. The degree of wear of said facesdetermines the mean time between failure of the motor which, in turn,depends on the wear resistance of the materials used for the frictioncoupling constituted of the operational face 5 of the oscillator 2 andthe surface of the rotor 6. The selection of the rotor material is noparticular problem. It can be selected both, from very hard materialssuch as ceramics based on alumina, zirconium oxide, titanium carbide,tungsten carbide, thermally treated alloy steel and the like, and fromsoft composite friction materials on the base of thermally reactiveplastic materials including solid fillers. The selection of theoscillator material is more difficult since the oscillator has to bemanufactured from piezoactive material. Only a limited number ofpiezoactive materials can be directly used. Such are quartz, somecrystallites, and barium titanate. Quartz has a very small piezoelectricmodule which limits its use. Barium titanate has a low Curie-point whichopposes its use over a wide temperature range. The problem of havingavailable a wide scope of piezoceramics has been solved in that theoperational face 5 of the oscillator 2 is provided with a thinwear-resistant layer which resists the frictional wear of the oscillator2 and determines the friction coefficient of the operational face 5 atthe same time. The material of friction layer 5 has to satisfy thefollowing conditions: it has to be resistant to the friction wear due tothe friction effect oscillator-rotor motor; it has to form a stable, forexample, chemical compound with the piezoceramics. Additionally, it hasto be resistant against strong ultra-sonic fields.

In FIGS. 18a-18e a number of modifications of a wear resistant layer 60for the piezoelectric motor is shown. According to FIG. 18a, the wearresisting layer can be a ring 61 made of a 0.1 to 0.3 mm thin oxideceramics (based on alumina or another material), which is cemented tothe piezoelectric oscillator 2 by means of a material which forms achemical compound with the oxide ceramics and the piezoceramics, forexample, glass. Such a glass has to contain a sufficient amount of leadoxide. In FIG. 18b a modification of a wear-resisting layer 60 isrepresented which, in the present case, is a thin layer of hardened andwear resisting glass applied to the operational face 5 of the oscillator2. The glass is applied to the the oscillator 2 before being polarized.In order to avoid splintering when cooling down the glass has to beselected based on temperature coefficient of expansion in such a mannerthat it does not differ from that of the piezoceramics by more than 5%.Such layers are employed in motors of low power.

The use of a friction layer of pure glass is disadvantageous due to thevery low friction coefficient. It is feasible to treat the glass with apowder 62 made of an abrasive material as shown in FIG. 18c to increasethe friction coefficient. Powders of alumina, titanium carbide, titaniumnitride or the like can be employed as abrasive fillers. Such anembodiment permits a precise selection of the wave resistance of thefriction layer 60 which renders them non-discernible to ultra-sonicwaves, i.e. the reliability of the compound is increased and, hence, theoutput of the motor.

Particular stable friction layers for piezoceramic stepping power motorscan be manufactured from porous oxide ceramics filled with a materialwhich forms a compound with the piezoceramics of the oscillator 2. Sucha friction layer 60 is shown FIG. 18d. A porous alumina ceramics is bestsuited for that, the pores of which are filled with an easily meltingglass material forming a chemical compound with the piezoceramics of theoscillator 2.

In FIG. 18e a modification of the friction layer 60 is represented as adouble layer structure constituted of a base layer 63 and anintermediate layer 64. In this embodiment the base layer determines thefriction properties of the operational face 5, the intermediate layer 64forming a stable connection between the piezoceramics and the oscillator2. It is technologically reasonable to apply the intermediate layer 64as a metal layer simultaneously forming the structure of the electrodes15, 16 of the oscillator 2. It is feasible to produce such a coat bychemically precipitating nickel, by sputtering nickel, tantalum or anyother material by means of ion implantation. The base layer 63 of chromeor any stable material can be applied to the intermediate layer 64 inthe range of the friction contact by means of a chemical orelectrochemical procedure. The present invention can also use anysuitable method for formation of the friction layer 60. Each embodimentof the oscillator 2 for the piezoelectric motor comprises a number oftravelling wave generators 4 which substantially occupy its entirevolume. Said generators determine the structure of the electrodes 15, 16on the surface of the wave guide 3 of the oscillator 2. FIGS. 19a-19eshows the most important embodiments of the electrodes 15, 16. In FIGS.19a an oscillator 2 is shown comprising a generator 4 and three groupsof electrodes 15, 16. Each of said groups of electrodes constitutes astanding wave generator 9, 10, 11 (FIG. 1). FIG. 19b shows an oscillatoras a short cylinder. Said oscillator has four travelling wave generators4. The entire electrodes 15 of said oscillator constitute a commonelectrode one the outside. The single electrodes 16 are provided on theinternal face of the oscillator 2. In FIG. 19c, the oscillator 2 is ofannular shape with the electrodes 15, 16 arranged on the plane faces.The FIG. 19d shows an embodiment of the oscillator 2 in which thestripe-shaped electrodes 15, 16 are arranged on the external face. Suchan oscillator 2 has a longitudinally polarized wave guide 3 face and canbe employed when the oscillator is excited by high voltages. In thisembodiment the oscillator 2 has no electrodes on its internal face. Theentire electrodes 16 of said oscillator 2 are connected in parallel andconstitute the common electrodes of the standing wave generators 9, 10,11. Accordingly, each of the electrodes 15 is coordinated to arespective generator 9, 10, 11.

FIG. 19e shows a developed oscillator 2 with a modification of theelectrodes 15, 16 in which the entire connections between the electrodesare embodied as lines 70. It is feasible to produce such a structurephotochemically. The invention provides for a particular configurationof electrodes for each standing wave generator 9, 10, 11 as shown inFIG. 20. In this embodiment the electrodes 15, 16 have a height c whichequals about half the height h of the oscillator 2 and are spaced apartby a distance l. In this case an elastic wave 700 is simultaneouslyproduced with the longitudinal wave 500. The amplitude ratio betweenlongitudinal wave and elastic wave can be varied by varying the heightof the electrodes 15, 16. The length,λ_(t) the elastic wave depends onthe height h of the oscillator. By varying the height c of theelectrodes 15, 16 and the height h of the oscillator 2 the values can beadjusted so that the amplitude and the length of the elastic wave equalsthe amplitude and the length λ_(l) of the longitudinal wave, as shown inthe wave diagram of FIG. 20. When both waves are simultaneouslygenerated in the oscillator 2 the deformations caused by themsuperimpose. At the bottom of FIG. 20 the entire deformations of theoscillator 2 are shown. It is rendered visible that the amplitude of thesuperimposed wave of the operational face 5 equals double the amplitudeof the longitudinal wave and that the longitudinal deformations and theelastic deformations mutually compensate on the leading face 7, that is,said leading face 7 is not deformed, no wave results.

It is obvious that, when generating three standing waves in theoscillator 2 by the three generators 9, 10, 11 in analogy to thegenerator shown in FIG. 21a, a non-symmetrical travelling wave isgenerated in the manner as shown in FIG. 21b. Said wave deforms theoperational face 5, the leading face 7 being not deformed. In thisembodiment of a piezoelectric motor the oscillator 2 energy is notabsorbed by the sound isolating base 8. Therefore, such motors have apotentially higher efficiency.

The invention provides different modifications for controlling thestanding wave generators 9, 10, 11 via the excitation sources 17, 18,19. FIGS. 22a-22c shows the circuits illustrating the general excitationprinciple for connecting the excitation sources 17, 18, 19 to thegenerators 9, 10, 11.

The circuit according to FIG. 22a exemplifies the separate excitation ofthe generators 9, 10, 11 via the sources 17, 18, 19. It is obvious thatthis circuit can be substituted for the circuit according to FIG. 22b inwhich all contacts 21 of the sources 17, 18, 19 are interconnected in acommon collective line 76 and the entire electrodes 15 of the generators9, 10, 11 are substituted for a common electrode 77. The collective line76 and the common electrode 77 are connected with one another via aconnection line 78. Such a modification does not change the generalexcitation principle, however, means a simplification of the circuit.Furthermore, it is feasible to supply equal electric potentialsphase-shifted by 2/3 π (120°) to one another, since the sources 17, 18,19 are equal in amplitude and frequency, and since the electricresistances of the generators 9, 10, 11 are equal to one another nocurrent flows through the connection line 78 and it can be omittedaccording to FIG. 22c. This includes that the common electrode 77 needsno electric contact.

A further simplification of the invention is feasible according to FIG.23 in which the oscillator 2 including the travelling wave generator 4is visible. Again the three standing wave generators 9, 10, 11 arerepresented, 9 being the basic generator and 10, 11 being boosters forthe standing wave generation. The generator 11 only is voltage suppliedin connection with the generators 9, 10, whereas the generators 9 and 10are directconnected to the voltage sources 17, 18. Therefore thegenerator 11 can be considered as a common portion of the generators 9,10 arranged between two areas m, n being split along an area s, and inwhich the portion between the areas m and s belongs to the generator 9and the portion between the areas s and n belongs to the generator 10.

The electrodes 15, 16 of the generators 9, 10, 11 are connected to theexcitation or voltage sources 17, 18, the latter supply them withvoltages U₁, U₂. In the equivalent circuit shown in FIG. 24 X₁, X₂, X₃are the impedances of each of the portions of the generators 9 and 10.When the generator portions 9 and 10 are equal to one another then theimpedances X₁, X₂, X₃ are equal. According to the invention the phaseangle between the voltages U₁, U₂, U₃ has to be ±120° or approximatelythat value. In this embodiment the voltages U₁, U₂ applied across theoscillator 2 are divided by the dividers X₁, X₂, X₃ and the resultingportions are summed up at X₃. The voltages U₁ ', U₂ ', U₃ ' result ateach portion of the oscillator 2. When X₁ =X₂ =X₃ then U₁ '=U₂ '. Thevoltage U₃ is slightly smaller than U₁ and U₂ respectively, and thevoltage U₃, smaller than U₁ and U₂ ', respectively. The phase shiftangle φ between the voltages U₁ and U₂ is about 120°. The phase anglebetween the voltages U₁ ' and U₂ ', respectively, and U₃ ' is about φ/2.Since the common portion 11 of the generators 9 and 10 is, considered bythe current passing them, connected in opposition to the separateportions, the phase of its excitation voltage is rotated by 180°; thatis, the phase φ' of the voltage U₃ ' which excites the second booster 11is 180°-φ/2; accordingly, when in this special case φ=120°, thenφ'=180°-120°/2 120°.

In this manner, in the present embodiment, the third standing wave isexcited by the common portion 11 of the two standing wave generators 9and 10. Said common portion is excited by a voltage formed bysuperposition of portions of the voltages U₁ and U₂. This is the reasonwhy there is no excitation source 19 in this embodiment, which means aconsiderable simplification. When the present invention is reduced topractice it is difficult, due to the separate excitation of thegenerators 9, 10, 11, to ensure a simple synchronous operation andin-phase-operation of the excitation sources 17, 18, 19. Therefore theembodiment according to FIG. 25 of the invention offers aself-excitation which does not change the feature of the invention. Inthe case of a self-excitation of the generators 9, 10, 11 the excitationsource 17 of the basic generator 9 represents the source for the carrierexcitation signal and the excitation sources 18, 19 are, practicallyspeaking, employed to rotate the phase and to enhance said signal. Inthis case, the excitation source 17 includes a generator 79 for theelectrical carrier signal and a power amplifier 80, the sources 18, 19include phase shift means 81, 82 and buffer amplifiers 83, 84.Additionally, the sources 18, 19 can include means 85, 86 for polereversal of the phase angle. Said embodiment operates as follows. Thecarrier signal generator 79 produces an electric signal the frequency ofwhich equals the mechanical resonant frequency f₀ of the oscillator 2.This signal is fed into the amplifier 80 and into the phase shift means81, 82 of the sources 18, 19. The phase shift means rotates the phase ofthe signal by ±2/3 π (±120°). From the phase shift means 81, 82 thesignal of each source 18, 19 is fed into the means 85, 86 for polereversal of the phase angle. Subsequently the signals are amplified inthe amplifiers 83, 84. The three signals amplified in the amplifiers 80,83, 84 are fed as excitation voltages of the sources 17, 18, 19 into therespective standing wave generators 9, 10, 11.

The frequency diagrams according to FIG. 16 of the piezoelectric motorare continuous, i.e. there are no points of unstable states. Hence, thefrequency of the control generator 79 can dwell in any range of thefrequency curve 42. If required, it is feasible to vary the rotationfrequency of the rotor 6 by varying the excitation frequency. By meansof the devices 85, 86 it is feasible to reverse the rotation directionof the rotor by reversing the phase of the signals from the sources 18,19 into the respective opposite direction (by ±120° related to the phaseposition of the signal from the source 17).

In FIG. 26 an embodiment of the motor is represented comprising acontrol generator 79 operating as a frequency controlled autogenerator.In said embodiment the rotation frequency of the rotor 6 can be variedby varying the control voltage U_(R), that is, by varying the excitationvoltage of the control generator 79. In many applications of the presentinvention it is very suitable that the operation frequency of the motoris substantially equal to the mechanic resonant frequency f₀ of theoscillator 2, due to possible destabilising effects. Such a modificationis shown in FIG. 27. According to said modification the motoradditionally includes a positive feedback branch 87 comprising an input88, a filter 89, a phase-shift chain 90 and an amplifier 91.Additionally, said modification includes an impedance 92series-connected to the basic generator 9, the excitation source 17which, in the present case, only includes the amplifier 80. The input 88of the feedback branch 87 connected via a line 880 to the electrodes 15,16 of the impedance 92 include in the basic generator 9. With dashedlines a second possible connection 881 to the impedance 92 is shown inFIG. 27. The output of the feedback branch 87 is connected to the bufferamplifier 80 of the source 17. The impedance 92 can be a resistor R, aninductor L, or a capacitor C. In its enity the considered modificationis an electromechanical autogenerator which is excited at the mechanicalresonant frequency f₀ of the oscillator 2. The auto-generator is reducedto practice by a basic generator 9 and its excitation source 17 and theadditional feedback branch 87, constituting a closed electromechanicchain. In order to excite said chain on the mechanical resonantfrequency f₀ of the oscillator 2 the phase diagram 43 of FIG. 16 of themotor is employed, having a zero-passage in the vicinity of thefrequency f₀.

The above described electromechanical chain is tuned in such a mannerthat the amplification factor at the frequency f₀ is greater than unityand the phase shift is zero between input signal and output signal atsaid frequency for each discontinuity of the chain; the amplificationfactor being preselected by the amplifiers 80, 83, 84 and the phaseshift by the phase shift chain 90. The size of the phase shift of thesignal depends on the component selected for impedance 92. In the caseof employing a resistor the phase shift only has to correct and lieswithin a range of ±10°. When inserting a capacitor C it lies in therange of +90° and when employing an inductor L it lies at -90°.

FIG. 28 shows an embodiment comprising a current transformer 93 in theinput circuit of the feedback branch 87. Said embodiment simplifies thecircuit for the electromechanical auto-generator for the case that theelectrode 77 output is omitted. In its entity the arrangement operatesin analogy to that of FIG. 27, considered hereinabove.

In a few applications which require a precise tuning of the excitationvoltage U₁ (U₂, U₃) to the mechanic resonant frequency f₀ of theoscillator 2 a modification is feasible which includes an additionalfeedback electrode 95 shown in FIG. 29a. Said electrode is arranged atthe locus of the mechanical stress maximum of the standing wave of thebasic generator 9. Two modes of operation of the electrode 95 arefeasible: no-load operation and short-circuit operation. In no-loadoperation, the electrode 95 is no resistance for the electrical load.When the generator 17 is excited a voltage proportional to themechanical stress of the standing wave of the basic generator 9 isproduced due to the direct piezoelectric effect on the electrode 95. Ashort-circuit current flows due to the low resistance in the case of theshort-circuit operation of the electrode 95 and the electrode 77. Saidcurrent is proportional to the mechanical stress of the standing wave ofthe basic generator 9. The frequency diagrams and the phase diagrams ofthe no-load voltage U_(S) and of the no-load current I_(S) are used forthe electromechanical auto-generator and associated feedback electrode95, according to FIGS. 26b-29d.

FIG. 30 shows an embodiment of the electromechanical auto-generator andassociated feedback electrode 95. The function of the electrode 95 ispre-determined by a resistor 99. The arrangement is tuned via theamplification factor of the feedback and via the phase shift, in analogyto FIG. 27. The present invention also provides for the application of apiezoelectric motor in step operation with a minimum start-stop time. Tothis end, according to FIG. 31, a switch 100 having a control input 101is provided in the feedback branch 87 for on-off switching the electriccircuit. When switching OFF, the switch 100 opens or closes the electricsignal circuit of the feedback for a short time. In both cases thecontrol of the electromechanical auto-generator of the motor isinterrupted at a minimum stop time. When switching ON the feedbackbranch 87 via the switch 100 the auto-generator is rapidly started. Theswitch control and, hence, the control of the step operation is carriedout via a pulse control voltage U₁ of optional period of time.

Hereinafter a number of embodiments of the mechanical and electricaldesign of the piezoelectric motor are described.

FIGS. 32 and 33 represent an embodiment of the piezoelectric motorincluding an oscillator 2 elongated along its longitudinal axis X-X andone (or two) travelling wave generator(s). A piezoelectric motor 200comprises a stator 1 and associated oscillator 2 which simultaneouslyconstitutes the wave guide 3 for the longitudinally travelling wave. Theoscillator 2 is arranged for free running on a mounting base 102.Provided that the mounting base 102 is made of metal, an insulatingbacking 103 has to be provided between the former and the oscillator 2in order to eliminate a short-circuit between the electrode 16 (77) andthe stator 1 of the motor. The oscillator 2 abuts against thesound-isolating support 8 via its non-functional leading face 7. Therotor 6, connected via an elastic sleeve 105 to a motor axle 106, ispressed against the operational face 5 of the oscillator 2 by means of aspring 104. The axle 106 is seated on a ball-bearing 107 in such amanner that it can freely move in longitudinal direction and,accordingly, can transmit the contact pressure executed by the spring104 to the rotor 6.

When a travelling wave is generated in the oscillator 2 the operationalface 5 of the oscillator 2 transmits a moment of rotation to the rotor 6and induces the latter to rotate. The rotation is transmitted to themotor axle 106 via the elastic sleeve 105. When a mechanical load isapplied upon the axle 106 the oscillator 2 produces a moment of rotationopposite to the moment of rotation acting upon the axle 106 and therotor 6. Since the oscillator 2 abuts via its leading face 7 against thesound isolating support 8, a rotation of the oscillator 2 is eliminateddue to the friction moment between support 8 and the leading face 7.Said friction moment always is greater than the moment of rotation ofthe motor in friction contact, therefore the oscillator 2 remains in aposition of rest. This embodiment exhibits a simple construction whichpermits a fast assembly. It permits manufacture of minute geometries.Since external diameters of only 2 to 3 millimeters are feasible, it iscompetitive with conventional electromotors.

FIG. 34 shows the motor comprising an annular oscillator 2 ofsufficiently great diameter (greater 120 mm) and a sufficiently largeoperation face 5 (15 to 20 mm width of the annular face). The oscillator2 offers space for more than ten travelling wave generators 4. Thus ahigh motor output is ensured. Accordingly, this embodiment requires asuitable heat conductivity of the mounting base 102 and the rotor 6 todissipate the heat produced in the oscillator 2 while operating. To thisend, preferably aluminium and its alloys are used. In this embodimentthe rotor 6 of the motor is coated with a thin friction layer 600 whichdetermines the friction properties of the rotor 6.

FIGS. 35 and 36 show a piezoelectric power motor 200. Said motorcomprises a rotor constituted of the two parts 108, 109. The part 108 issecured to the axle 106. The part 109 is displaceable along the axle106. Both rotor parts are pressed against two conical operational faces110 of the cylindrical oscillator 2 via an elastic support 111 which, inturn, is pressed between the part 109 and a disk 113 by a nut 112. Thematerial of the elastic support can be selected from elastic synthetics,such as polyurethane. The oscillator 2 of said motor is installed in ahousing 114 illustrated by a bracket provided with a clamp bolt 115. Themotor has no bearing. The rotor 108, 109 is centered by the conicaloperational faces 110 of the oscillator 2. This construction permits amaximum output of the piezoelectric motor. The output is only limited bythe dynamic strength of the piezoceramic oscillator 2.

The present invention permits construction of a still more powerfulpiezoelectric motor in which a metallic wave guide is used. A feasibleembodiment of such a motor 200 is shown in the FIGS. 37 and 38. Itcomprises the stator 1 and the oscillator 2 with associated metallicwave guide 3. The wave glide 3 is made of a stable thermally treatedsteel permitting maximum mechanical stresses in the wave guide. Theseare considerably greater for steel than for piezoelectric ceramics. Sucha motor permits maximum output per volume unit of the wave guide. In thepresent design the standing wave generators are manufactured in theshape of packets of piezoelectric transducers 116 compressed by bolts117. They are coaxially arranged about the axle 106 and about thewaveguide 3 and are connected to the latter via a flange 118.

Such a motor operates as follows: each group of packets of piezoelectrictransducers, constituting standing wave generators 9, 10, 11, producesstanding waves 26, 27, 28 (FIG. 7) in the wave guide 3. Thesuperposition of said waves produces the travelling wave 29 whichrotates the rotor 6. Such a construction of a piezoelectric motor,including piezoelectric transducer packets, permits unlimited toincrease of the mechanical output of the motor by multiplying the numberof packets in accordance with the diameter of the wave guide. Eachpacket having a diameter of 30 mm has a maximum transducer output ofabout 100 watt. With thirty packets on the oscillator 2 and anefficiency of 30% to 40% of the piezoelectric motor a mechanical outputof about 100 watt is obtainable on the motor axle 106.

In accordance with the requirements of a reduction to practice theinvention permits a number of modifications of the piezoelectric motor.

FIG. 39 shows a modification of a motor adapted for use in specialoptical systems. The motor has a wide central opening 231. The rotor 6again is pressed against the oscillator 2 by an annular magnet 119which, in turn, is attracted by the magnetic housing 120.

The invention offers various embodiments of the excitation sources ofthe standing wave generate 9, 10, 11 of the oscillator 2, four of themcontaining the most important components being described hereinafter.

FIG. 40 shows schematic electric circuit diagram of an embodiment of themotor which, by the frequency control principle, controls the rotationalspeed of the rotor. This circuit corresponds to the block diagrams ofFIGS. 25 and 26. The circuit comprises the source 17 of the basicgenerator 9, including the control generator 79 and the power amplifier80. The control generator 79 is set up by the auto-generator principlewith a Wien-bridge. It is feasible to vary the frequency of the controlgenerator 79 via the control voltage U_(R) which affects a varactor 122varying the capacitance of the same. The capacitance of the varactor 122is selected in a manner to vary via its variation the frequency of thecontrol generator 79 in the range of the frequency curve shown in FIG.16a. In the present embodiment the sources 18, 19 include the boosters10, 11, the phase shift means 81, 82, the means for pole reversal of thephase angle 85, 86, and the power amplifiers 83, 84. The phase shiftmeans 81, 82 are constituted of active phase shift branches 123, 124including operation amplifiers which rotate the signal phases by +2/3 π(120°) and -2/3 π (-120°), respectively. The means 85, 86 connect thechange-over switches 125, 126 to earth, the latter carry out the changeof function of the phase shift means 81, 82, that is, reverse the signof the phase shift. Such a reversal permits reversal of the rotationdirection of the rotor 6. The amplifiers 80, 83, 84 are identical onesand operate as buffer amplifiers (power amplifiers) having a broadtransmission band and nearly operate in the switching mode. The deviceconsidered reliably operates both, on the left and on the right of themechanical resonant frequency f₀ of the oscillator 2.

In FIG. 41 a basic circuit diagram according to the block diagram inFIG. 27 (auto-generator) is represented. In the circuit the input 88 ofthe feedback branch 87 is connected to the impedance 92 embodied asresistor R_(i). The voltage across this resistor R_(i) is proportionalto the current flowing through the basic generator 9. The phase shift ofsaid voltage at the frequency f₀ practically is zero, related to thevoltage across the generator 9. From the resistor R_(i), the voltageproportional to the current is applied across the band-pass filter 89(L_(f), C_(f)) and then to the amplifier 91. The band-pass filter 89,tuned to the frequency f₀, limits the filter range of the feedbackbranch 86 and thus eliminates any self-excitation of the circuit in therange of parasitic frequencies of the oscillator 2. The voltageamplified in the amplifier 91 is fed into the output of the feedbackbranch 87 and from there into the power amplifier 80 of the source 17 ofthe basic generator 9. Since, in the closed circuit, the overall phaseshift is zero at the frequency f₀, the amplification factor at thisfrequency is greater unity, and the source 17, including the feedbackbranch 87, starts to oscillate at the frequency f₀ and, hence, operatesas an auto-generator maintaining said frequency irrespective ofdestabilising affects. The phase shift means 81, 82 include the phaseshift branches 123, 124 which, in turn, include two mutually reversiblephase chains 127, 128. The reversal is carried out by change-overswitches 129 and 130 which practically cause a reversal of sign of thephase shift. Such a reversal permits to reverse the rotation directionof the rotor 6. The power amplifiers 80, 83, 84 are connected to thegenerators 9, 10, 11 via the separating filter L_(f) C_(f). Thus, it isfeasible to operate the amplifiers in the switching mode, the outputvoltages having steep slopes.

FIG. 42 shows an embodiment in which the capacitor C_(F) of theseparating filter L_(F) C_(F) is employed as impedance 92. The voltageamplitude across this capacitor is proportional to the current throughthe generator 9. The phase, however, is shifted by -90° relative to thegenerator current. Such a phase shift requires a further shifting by-90°. This shift is performed in the capacitor circuit C_(f) of theband-pass filter 89 where the voltage is shifted by -90° at the resistorR_(f) relative to the capacitor current. Accordingly, the total phaseshift is -180°. The amplifier 91 again shifts the phase by -180° so thatthe total phase shift is zero. Since, with that embodiment, it is notfeasible to reverse the rotation direction of the rotor 6 the means 85,86 are omitted. In the embodiments of the operational components of themotor 200, shown in FIGS. 40, 41, 42 half-bridge circuits includingbipolar or field-effect transistors (not shown) are used as bufferamplifiers 80, 83, 84 operating as voltage control. Such circuitsprovide electric voltage amplitudes at the generators 9, 10, 11 equal tohalf the supply voltage. Provided that higher voltages are required,circuits including current change-overs can be employed. Such circuitsprovide a voltage at the generators which is 2 to 3-fold higher thevoltage supplied by the power amplifiers.

FIG. 43 shows an embodiment of the motor comprising motor comprisingcurrent switches. Said embodiment includes three current reversecircuits 131 supplied by a voltage E and embodied as bipolar transistorswith a current source set-up by an inductor L_(I) in the collectorcircuit. The voltage amplitude at the generators 9, 10, 11 is about(2-3) ·E. In this embodiment, the feedback branch 87 is connected to thefeedback electrode 95 which, at the frequency f₀, generates a phaseshift of +90° between the voltage across the generator 9 and the voltageacross said electrode 95 (FIG. 29, position 97). The phase shift chain90 of the band-pass filter 89 rotates the phase again by +90° so thatthe total phase shift is +180°. The rotation of the phase in oppositedirection by -180° is performed by the amplifier 91. This modificationrepresents a piezoelectric motor operating in step mode having a shortstart-stop-time, set-up in accordance with the block diagram of FIG. 31.To provide for step operation, the circuit includes a transistor switch100 controlled via the input 101 pulse voltage. When the switch 100 isclosed, the feedback is split up causing a rapid stop of theoscillations of the electromechanic generators. When the switch 100 isopened, the auto-generator is rapidly started.

FIG. 44 shows an electric circuit of the motor in which the portions ofthe voltages U₁ and U₂ exciting the generators 9, 10 are summed up. Thecircuit is set-up in a manner that, when actuating a change-over switch130, the excitation sources 17, 18 and the generators 9, 10 areoperationally exchanged. This, in a simple way, permits the reversal ofthe rotation direction of the motor. As concerns further components, thereference numerals used hereinbefore are valid. In the entire feasibleembodiments of the piezoelectric motor, the piezoelectric ceramicoccupies the maximum obtainable volume of the oscillator. Therefore suchmotors are distinguished by a highly effective oscillator of a maximumelectromechanic coefficient. They only require a low excitation voltageand permit high loads, that is, they also work when the oscillator issubject to high mechanical stress. The monolithic oscillator of suchmotors has a high stability which permits a maximum mechanical stressand, hence, a maximum oscillation speed which, in turn, ensures amaximum mechanic output of the motor axle.

Thus, the motor provides, for example, a specific mechanical output of10 W/cm² operational face at a specific tangential load on theoperational face of the oscillator of 10 N/cm² and at an oscillationspeed of the operational face of 1 m/s. That is, the attainable maximummechanical output of the motor is 450 W at an oscillator diameter of 100mm and at an operational face width of 15 mm.

The high mechanical stability of the oscillator ensures the highreliability and the long life of the motor which approximates thereliability of solid components. The simple design reduces the net costswhich renders said motor competitive to conventional electromotors.

List of Reference Numerals

1--stator

2--oscillator

3--waveguide

4--travelling wave generator

5--operational face of the oscillator 2

6--rotor

7--leading face of the oscillator 2

8--sound isolating support

9--basic generator for standing waves

10, 11--boosters (additional generators) for standing waves

12, 13, 14--oscillator sections

15 16--electrodes of standing wave generators

17, 18, 19--excitation sources, (voltage sources)

20, 21 contacts of excitation sources

22--central line of oscillator

23, 24, 25--deformation diagrams of operational face 5

26, 27, 28--deformation states of operational face 5

29--travelling wave curve

30, 31, 32, 33--path of travel of points

34--elementary volume of oscillator

35--elementary oscillation system

36--elementary equivalent circuit

37, 38, 39--equivalent circuits of standing wave generators

40--friction transducer

41-46--electromechanic motor parameters

47-54--oscillator embodiments

55-64--modification of friction layers

60-69--arrangements of electrodes

70--lines

71--special arrangement of electrodes

72--developed oscillator (schematic view) including special electrodearrangement

73, 74, 75--block diagram

76--collective conductor

77--common electrode

78--connection line

79--control generator

81, 82--phase shift means

80, 83, 84--buffer amplifiers

85, 84--means for pole reversal of phase angle

87--feedback branch

88--input of feedback branch

89--band-pass filter

90--phase shift chain

91--amplifier

92--impedance

93--current transformer

94--part oscillator

95--feedback electrode

96-98--characteristic of feedback electrode

99--resistor

100--switch

101--control input

102--mounting base

103--insulating backing

104--spring

105--elastic sleeve

106--axle

107--ballbearing

108, 109--portions of the rotor

110--conical operational faces

111--elastic support

112--nut

113--washer

114--housing

115--jolted bolt

116--packets of transducer

117--bolt

118--flange

119--annular magnet

120--magnetic conductive housing

121--autogenerator with wien-bridge

122--capacitor

123, 124--phase-shift branches

125, 126--change-over switches

127, 128--phase chains

129, 130--change-over switches

131--current reverse circuit

150, 151--arrows

200--piezoelectric motor

231--opening

500, 700--waves

600--friction layer

880--conductor

881--possible connection

X-X--axis

I claim:
 1. A piezoelectric motor comprising:a rotor; a stator in theform of hollow cylindrical waveguide formed of a piezoelectric materialwhich is homogeneously polarized uniformly in a polarization directionwhich is in a radial direction of said hollow cylindrical waveguide,said hollow cylindrical waveguide having a first end surface, a secondend surface, an external cylindrical surface and an internal cylindricalsurface; said rotor being in contact with said first end surface of saidhollow cylindrical waveguide; generators for generating an longitudinaltraveling wave, in the form of an expansion-compression traveling wavehaving a wave length λ and traveling circumferentially on said first endsurface, said longitudinal traveling wave being generated fromexpansion-compression standing waves on said first end surface generatedsaid generators; each of said generators including:a first generator forproducing a first expansion-compression standing wave of saidexpansion-compression standing waves, said first generator includingfirst and second electrodes respectively on said external cylindricalsurface and said internal cylindrical surface which extendperpendicularly to said polarization direction and oppose one anotherand are disposed on a first sector of said hollow cylindrical waveguidehaving a circumferential length of λ/3; a second generator for producinga second expansion-compression standing wave of saidexpansion-compression standing waves, said second generator includingfirst and second electrodes respectively on said external cylindricalsurface and said internal cylindrical surface which extendperpendicularly to said polarization direction and oppose one anotherand are disposed on a second sector of said hollow cylindrical waveguidehaving a circumferential length of λ/3; a third generator for producinga third expansion-compression standing wave of saidexpansion-compression standing waves, said third generator includingfirst and second electrodes respectively on said external cylindricalsurface and said internal cylindrical surface which extendperpendicularly to said polarization direction and oppose one anotherand are disposed on a third sector of said hollow cylindrical waveguidehaving a circumferential length of λ/3; said first, second and thirdsectors being disposed adjacent one another and together forming agenerator sector having a circumferential length of λ; and energizingmeans for driving said first and second electrodes of said first, secondand third generators such that said first, second and thirdexpansion-compression standing waves are phase shift from each other by2 π/3 to produce said expansion-compression traveling wave having saidwave length λ to rotate said rotor by means of frictional contactbetween said first end surface and said rotor; and said hollowcylindrical waveguide having a circumferential length equal to N numberof said wave length λ of said expansion-compression traveling wavewherein N is an integer equal to at least one, and said generatorsincluding N number of said generators with said generator sectorsthereof disposed adjacent one another.
 2. The piezoelectric motor asclaimed in claim 1, wherein one of said first and second electrodes ofsaid first, second and third generators are formed in common.
 3. Thepiezoelectric motor as claimed in claim 1, wherein N equals 2 and saidgenerators are uniformly distributed over said hollow cylindricalwaveguide.
 4. The piezoelectric motor as claimed in claim 1,wherein:said energizing means include first and second voltage sourcesfor applying voltages to said first, second and third generators; andsaid first, second and third generators are interconnected to functionas a voltage divider with respect to the voltages delivered by saidfirst and second voltage sources, and a summator with respect to thevoltages divided.
 5. The piezoelectric motor as claimed in claim 1 or 4,wherein said hollow cylindrical waveguide is made of piezoelectricceramics.
 6. The piezoelectric motor as claimed in any one of the claims1, 3 and 4, wherein said first end face of said hollow cylindricalwaveguide is coated with a friction layer contacting said rotor andwhich is wear resistant.
 7. The piezoelectric motor as claimed in claim6, wherein said friction layer is entirely made of a material forming achemical compound with a material of said hollow cylindrical waveguide.8. The piezoelectric motor as claimed in claim 6, wherein the frictionlayer includes a basic layer and an intermediate layer, said basic layerdetermining friction properties and the intermediate layer forming achemical compound with materials of said hollow cylindrical waveguideand the basic layer.
 9. The piezoelectric motor as claimed in claim 6,wherein the friction layer is made of a composite material, a base ofsaid material forming a chemical compound with material of said hollowcylindrical waveguide to which a filler material is added to increase afriction coefficient of the friction layer.
 10. The piezoelectric motoras claimed in claim 6, wherein the friction layer is made of a porousmaterial of high friction coefficient, and pores of said porous materialare filled with a material different from said porous material and whichforms a chemical compound with a material of said hollow cylindricalwaveguide.
 11. The piezoelectric motor as claimed in any of the claims1, 3 and 4, wherein said energizing means includes a first driver havinga voltage signal generator for driving said first generator and secondand third drivers each including power amplifiers and phase shiftersdriven by said voltage signal generator for respectively driving saidsecond and third generators.
 12. The piezoelectric motor as claimed inclaim 11, wherein said second and third drivers are provided with meansfor pole reversal of a phase angle.
 13. The piezoelectric motor asclaimed in claim 11, wherein said voltage signal generator is afrequency-controlled voltage generator.
 14. The piezoelectric motor asclaimed in claim 1, wherein said energizing means is provided with apositive feedback branch from said generators to form an electromechanicauto-generator.
 15. The piezoelectric motor as claimed in claim 14,wherein the positive feedback branch is connected to an impedance memberbeing series-connected to said first generator.
 16. The piezoelectricmotor as claimed in claim 14, wherein the positive feedback branch isconnected to a current reversal circuit being series-connected to thefirst generator.
 17. The piezoelectric motor as claimed in claim 14,wherein the positive feedback branch is connected to a feedbackelectrode.
 18. The piezoelectric motor as claimed in claim 14, whereinan electronic switch including a control input for breaking-OFF saidpositive feedback branch is provided.